The diaphragm isolating the positive electrode active material and the negative electrode active material in alkaline zinc manganese battery, silver oxide battery, zinc-air battery, nickel-metal hydride battery and lithium ion battery needs to have a wide variety of properties, such as:
(1) preventing internal short circuit caused by the contact between the positive electrode active substance and the negative electrode active substance, but allowing ions can smoothly pass.
(2) having evenly distributed fibers to ensure the uniform and stable discharge performance of batteries.
(3) having appropriate tensile strength and alkaline absorption speed to meet the requirements of pipe manufacture process and continuous production line production process.
(4) having such alkaline absorption rate that it can meet the demand of ionic conductivity at the continuous discharge time, in particular having a large alkaline absorption rate at high-power discharge time.
(5) having excellent electrolyte resistance, especially under hot alkaline conditions, having stable dimension in the electrolyte, and preventing deformation or short circuit to ensure the stability of capacitance during the shelf life.
The phenomenon of “dendritic crystal” is widespread in alkaline manganese battery, however, the “dendritic crystal” easily penetrates the diaphragm with a large pore size, making the active materials of the two electrodes contact each other, thus generating internal short circuit and severely reducing the capacity of the battery. To prevent the internal short circuit caused by “dendritic crystal”, it is required that the diaphragm should have smaller void structure, more specifically, the average pore size of the diaphragm should be less than 10 μm. Meanwhile, because the zinc power in the negative material of alkaline zinc manganese battery has a minimum diameter of 20 μm, the number of diaphragm pore whose size is greater than 20 μm is preferably not exceed 1% of the total number of pores, thus the internal short circuit can be prevented better.
Battery diaphragm generally is prepared by blending a variety of fibers through dry non-woven fabrics process or wet non-woven fabrics process, mainly has two types of structures, i.e., single layer and double layer. For example, the PCT patent application owned by Kuraray CO., LTD. (PCT publication No. WO2006/090790) discloses a single layer diaphragm used for alkaline battery, which has 25 to 62 wt % of alkali-resistant synthetic fiber, 5 to 25 wt % of fibrillated solvent-spun cellulose fiber with a Canadian standard freeness (CSF) of 10 to 280 ml, and 33 to 50 wt % of mercerized pulp with a CSF no less than 550 ml. Although such diaphragm has good alkali resistance and good compactness, good compactness increases the resistance of the diaphragm, deteriorates the absorbency of electrolyte and slows the absorption rate, and decreases the discharge performance of a battery. The PCT patent application owned by Eveready Battery Company, Inc. (PCT publication No. WO2003/043103) discloses another single layer battery diaphragm, which has a basis weight of 20 to 30 g/m2, a thickness less than 0.15 mm and an average pore size less than 14 μm, and consists of 25 to 95 wt % of fibrillated fibers and synthetic fibers with a beating degree of 30 to 65 Shopper Riegler (SR). Although having a small average pore size and good insulating performance, it also exists some shortcomings such as the poor absorbency and absorption rate of the electrolyte. In addition, a patent application for invention owned by NIPPON KODOSHI CORP (publication No. CN1177843A, with a priority of JP265147/96) discloses a diaphragm laminated by a certain liquid penetration layer and a certain compact layer. The compact layer consists of 20 to 80 wt % of the alkali-resistant cellulose fibers and synthetic fibers, wherein the beating degree of the cellulose fibers is 500˜0 ml when represented by CSF; the liquid penetration layer consists of 20 to 80 wt % of alkali-resistant cellulose fibers and synthetic fibers with a beating degree of 700 ml when represented by CSF; the compact layer forms alone. But this technology is difficult to meet the function of both having excellent electrolyte absorption performance and preventing internal short circuit at the same time. To guarantee the excellent electrolyte absorption performance, the relatively larger amount of liquid penetration layer will be required, and the amount of compact layer will be reduced at the moment. Because the compact layer forms alone and has less basis weight, the defects of the diaphragm are inevitably more, and the number of big pores increase, while the liquid penetration layer can not compensate those big pores, making it difficult to achieve excellent isolation performance. If increasing the amount of compact layer, the amount of liquid penetration layer will be reduced, then the diaphragm will be difficult to achieve excellent electrolyte absorbency. Such laminating mode makes the poor interlayer bonding force between the liquid penetration layer and the compact layer of diaphragm, and the large shrinkage during use.